Quaternary amine functionalized chitosan for enhanced adsorption of low concentration phosphate to remediate environmental eutrophication

Phosphate contamination of natural waterways from agricultural runoff is a major global issue due to its widespread occurrence and severe environmental effects, namely accelerated eutrophication, and subsequent negative impacts on aquatic ecosystems. This study reports on the synthesis of a biopolymer capable of remediating environmentally relevant concentrations of phosphate: a glutaraldehyde crosslinked chitosan, quaternised with 2-chloro-N,N-diethylethylamine hydrochloride (DEAE). This modification generated a notable increase in the overall positive surface charge on the chitosan promoting adsorption of phosphate anions. The chemical transformations on the adsorbent's surface are evidenced by the significant shift in zeta potential, increasing from –12 eV to +27 eV at pH 7, and analysis of the XPS spectra showed a substantial increase in the N1s peak centred at 402 eV, corresponding to the positively charged amine species. Langmuir isotherm modelling indicated the maximum capacity for phosphate to be 20.1 mg P/g and a Langmuir constant of 1.15 L/mg. Other adsorption studies were designed to focus on the identified issue of low concentration phosphate removal and demonstrated the modified-chitosan to be a highly effective adsorbent: using an adsorbent dose of 2 g/L resulted in 97% removal, leaving less than 0.1 ppm P in solution. A higher removal, 98.4%, was achieved at a dose of 6 g/L, however this represented considerable diminishing returns. Furthermore, there was no measurable loss of the intrinsic adsorption ability after 9 adsorption-desorption cycles showing exceptionally effective regeneration using 0.1 M NaCl.